Problem: $ E = \left[\begin{array}{rrr}0 & 1 & 2 \\ 1 & 2 & 1 \\ 0 & 0 & 1\end{array}\right]$ What is $ E^{-1}$ ?
Answer: $ = \left[\begin{array}{rrr}-2 & 1 & 3 \\ 1 & 0 & -2 \\ 0 & 0 & 1\end{array}\right]$